Samer ShammaaTelecommunications Eng. Dept.

Dr. Pramode Verma

OutlineIntroductionWSN ApplicationsTopologiesArchitecturePhysical LayerMAC LayerSuperframe structure

IntroductionUntil recently, the main concern in wireless

communication was on high throughputSome applications need a different set of

requirementsExample: LR-WPAN applications-Low cost communication network-Limited power-Low throughput Require: reasonable battery life, extremely low

cost, short range operation, reliable data transfer

LR-WPAN ApplicationsHome Automation

Heating, ventilation, air conditioning, security, lighting, control of objects.

Industrial Detecting emergency situations, monitoring machines.

Automotive Automotive sensing such as time pressure monitoring.

Agriculture Sensing of soil moisture, pesticide, herbicide, PH levels.

Others Controlling consumer electronics, PC peripherals, etc.

Solution?Existing standards not suitable for these

applications b/c of complexity, power consumption, and high cost.

Need a simple, flexible protocolIEEE 802.15.4 defines protocol via RF for

PAN.Provide a standard with ultra-low complexity,

cost, and power for low-data-rate wireless connectivity among inexpensive fixed, portable, and moving devices.

Wireless Protocols Comparison

Comparison (2)

Device TypesFull function device (FFD)

Any topologyPAN coordinator capableTalks to any other deviceImplements complete protocol set

Reduced function device (RFD)Limited to star topology or end-device in a

peer-to-peer network. Cannot become a PAN coordinatorVery simple implementationReduced protocol set

Modes of OperationNetwork Device: An RFD or FFD

implementation containing an IEEE 802.15.4 medium access control and physical interface to the wireless medium.

Coordinator: An FFD with network device functionality that provides coordination and other services to the network.

PAN Coordinator: A coordinator that is the principal controller of the PAN. A network has exactly one PAN coordinator.

Network Topologies

Combined TopologyEx: hotel

where cluster nodes exist between the rooms of a hotel and each room has a star network for control.

Star Network FormationAfter an FFD is activated, it can establish its

own network and become the PAN coordinator

Choose a PAN Identifier different from surrounding networks (within RF sphere of influence)

The PAN coordinator allows other devices, potentially both FFDs and RFDs, to join its network.

Peer-to-peer Network FormationEach device is capable of communicating with any

other device within its radio sphere of influenceOne Device is nominated as the PAN coordinatorForm first cluster by choosing an unused PAN

identifier and broadcasting beacon frames to neighboring devices.

A candidate device receiving a beacon frame may request to join the network at the PAN coordinator.

If the PAN coordinator permits the device to join, it adds the new device as a child device in its neighbor list.

Continued…Newly joined device adds the PAN coordinator

as its parent in its neighbor list and begins transmitting periodic beacons

Other candidate devices may then join the network at that device.

Once predetermined application or network requirements are met, the first PAN coordinator may instruct a device to become the PAN coordinator of a new cluster adjacent to the first one.

Other devices gradually connect and form a multi-cluster network structure

Cluster Tree Network

Architecture

Physical LayerProvides two

services:-PHY data service -PLME-SAP providing

data and management services to upper layers.

PHY Data ServiceEnables the transmission and reception of

PHY protocol data units (PPDUs) across the physical radio channel

Activation and deactivation of the radio transceiver

Energy detection within the current channelLink quality indication for received packetsClear channel assessment for CSMA-CAChannel frequency selectionChannel switching

Operating Band2.4GHz band operates worldwide- offers

250kb/s868 MHz band operates in Europe- offers 20

kb/s915 MHz band operates in United States-

offers 40kb/s

PHY Frame Structure

The 32-bit preamble is used for synchronization

“11100101” indicates start of packet7 out of the 8 PHY header bits are used to

indicate the length of the PSDUThe PSDU has a variable length between 0

and 127 bytes

MAC LayerThe MAC sublayer provides two services: -MAC data service: enables the transmission

and reception of MAC protocol data units (MPDUs) across the PHY data service

-MLME-SAP: provides data and management services to upper layers

MAC Sublayer FeaturesBeacon managementChannel accessGuaranteed Time Slot (GTS) managementFrame validationAcknowledged frame deliveryAssociationDisassociationProvides means for implementing application-

appropriate security mechanisms

MAC Frame Format

Frame Control Field

MAC Frame TypesIEEE 802.15.4 defines 4 types of MAC

frames:Beacon FrameData FrameAcknowledgment FrameMAC Command Frame

Beacon Frame Format

Data Frame Format

Acknowledgment Frame Format

Command Frame Format

Association requestAssociation

responseDisassociation

notificationData request

PAN ID conflict notification

Orphan NotificationBeacon requestCoordinator

realignmentGTS request

Superframe Structure

The active portion is divided into 16 equally sized slots

During the inactive portion, the coordinator may enter a low-power mode

The beacons are used to synchronize the attached devices, to identify the PAN, and to describe the structure of the superframes

Superframe Structure

Guaranteed Time Slots (GTSs)For low-latency applications or applications

requiring specific data bandwidthPAN coordinator may dedicate portions of the

active superframe to that applicationPAN coordinator may allocate up to seven of

these GTSs, and a GTS may occupy more than one slot period

Questions?

Data TransferThree types of data transfer:-Data transfer to a coordinator in which a

device transmits the data-Data transfer from a coordinator in which the

device receives the data-Data transfer between two peer devices*In star topology only first two are used*The mechanisms for each transfer type

depend on whether the network supports the transmission of beacons

Data Transfer to a Coordinator

Beacon-enabled PANSlotted CSMA-CA

Nonbeacon PANUnslotted CSMA-CA

Data Transfer from a Coordinator

PAN indicates message is pending in the beacon frame

Device request data at application-defined rate

Peer-to-peer Data TransfersDevices wishing to communicate will need to

either receive constantly or synchronize with each other

In the first case, the device can simply transmit its data using unslotted CSMA-CA

In the latter case, other measures need to be taken in order to achieve synchronization

Improving Probability of Successful DeliveryThe IEEE 802.15.4 LR-WPAN employs

various mechanisms to improve the probability of successful data transmission:

CSMA-CA mechanismFrame acknowledgmentData verification

Unslotted CSMA-CA MechanismUsed by nonbeacon-enabled PANs Each time a device wishes to transmit data frames

or MAC commands, it waits for a random periodIf the channel is found to be idle, following the

random backoff, the device transmits its dataIf the channel is found to be busy following the

random backoff, the device waits for another random period before trying to access the channel again

Acknowledgment frames are sent without using a CSMA-CA mechanism

Slotted CSMA-CA MechanismUsed by beacon-enabled PANsBackoff slots are aligned with the start of the

beacon transmissionDevice locates the boundary of the next backoff slot

and then waits for a random number of backoff slots

If the channel is busy, following this random backoff, the device waits for another random number of backoff

If the channel is idle, the device begins transmitting on the next available backoff slot boundary

Frame AcknowledgmentA successful reception and validation of a data

or MAC command frame can be optionally confirmed with an acknowledgment

If the originator does not receive an acknowledgment after some period, it assumes that the transmission was unsuccessful and retries the frame transmission

When the acknowledgment is not required, the originator assumes the transmission was successful

Data Verification-FCS MechanismIn order to detect bit errors, an FCS

mechanism employing a 16-bit International Telecommunication Union—Telecommunication Standardization Sector (ITU-T) cyclic redundancy check (CRC) is used to detect errors in every frame

Approaches for Low PowerThe protocol has been developed to favor

battery-powered devicesBattery-powered devices will require duty-

cycling to reduce power consumptionThus will spend most of their operational life in a

sleep stateEach device periodically listens to the RF

channel in order to determine whether a message is pending

Higher powered devices have the option of listening to the RF channel continuously

Service PrimitivesThe services of a

layer are the capabilities it offers to the user in the next higher layer or sublayer by building its functions on the services of the next lower layer

Continued…Each event consists of passing a service

primitive from one layer to the other through a layer SAP associated with an N-user

Service primitives convey the required information by providing a particular service

A service is specified by describing the service primitives and parameters that characterize it

Primitive Types— Request: The request primitive is passed from the N-

user to the N-layer to request that a service is initiated.— Indication: The indication primitive is passed from the

N-layer to the N-user to indicate an internal N-layer event that is significant to the N-user. This event may be logically related to a remote service request, or it may be caused by an N-layer internal event.

— Response: The response primitive is passed from the N-user to the N-layer to complete a procedure previously invoked by an indication primitive.

— Confirm: The confirm primitive is passed from the N-layer to the N-user to convey the results of one or more associated previous service requests.

Data Transfer Message Sequence

Indirect Data Transfer

Association

Disassociation

Data Polling-No Data Pending

Data Polling-Data Pending

References-IEEE Std. 802.15.4™-2006-Marco Naeve, Eaton Corp., IEEE 802.15.4

MAC Overview, 05/2004-IEEE Std. 802.15.4 Enabling Pervasive

Wireless Sensor Networks, Dr. Jose Gutierrez, Eaton Corp.

-http://deneb.cs.kent.edu/~mikhail/classes/seminar.u04/praveen_lrwpan.ppt

- 140.117.169.69/course1/zigbee-802.15.4.ppt

Questions?